mlx5: fix mlx5_get_vector_affinity to start from completion vector 0

The consumers of this routine expects the affinity map of of vector
index relative to the first completion vector. The upper layers are
not aware of internal/private completion vectors that mlx5 allocates
for its own usage.

Hence, return the affinity map of vector index relative to the first
completion vector.

Fixes: 05e0cc84 ("net/mlx5: Fix get vector affinity helper function")
Reported-by: Logan Gunthorpe <logang@deltatee.com>
Tested-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Cc: <stable@vger.kernel.org> # v4.15
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>